This study demonstrated the presence in infarcted dog myocardium of a number of polypeptides with affinity for heparin sepharose, the larger of which immunoreacted was a polyclonal antiserum to the aminoterminus of basic fibroblast growth factor. The smaller bands on the SDS gel, approximately 14,000 molecular weight, were mitogenic but not immunoreactive, possibly suggesting that they represent aminoterminus cleavage of fibroblast growth factor which is consistent with recent reports of protease cleavage. Such cleavage is enhanced in acidic conditions such as are found in the infarcted myocardium. That only limited cleavage, without loss of mitogenic capacity, occurs in myocardial infarction, may be important in understanding the angiogenic and wound healing responses in myocardial infarction.